sys-devel/llvm-3.9.0-r1 as well as sys-devel/llvm-3.9.1 fail building:
ninja: build stopped: subcommand failed.

• ERROR: sys-devel/llvm-3.9.1::gentoo failed (compile phase):
• (no error message)

Unfortunatly, masking triggers autounmask-write, so this jams updating the whole system.

Unable to attach build log due to "Cannot attach file build-abi_x86_32.x86.log: Unknown server error (413)". File has 3.4 MB, maybe that is to much? Will try updating later. For now, I appended the build-abi_x86_64.amd64.log